Cx51 User's Guide

Small Model Example

In the small model, parameters passed in fixed memory locations are stored in internal data memory. The parameter passing segment for variables is located in the data area.

The following are two assembly code examples. The first shows how the example function is called from assembly. The second example displays the assembly code for the example function.

Calling a C function from assembly.

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EXTRN CODE    (_function)         ; Ext declarations for function names
EXTRN DATA    (?_function?BYTE)   ; Seg for local variables
EXTRN BIT     (?_function?BIT)    ; Seg for local bit variables
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        MOV   R6,#HIGH intval                ; int  a
        MOV   R7,#LOW  intval                ; int  a
        MOV   R7,#charconst                  ; char b
        SETB  ?_function?BIT+0               ; bit  c
        MOV   ?_function?BYTE+3,longval+0    ; long d
        MOV   ?_function?BYTE+4,longval+1    ; long d
        MOV   ?_function?BYTE+5,longval+2    ; long d
        MOV   ?_function?BYTE+6,longval+3    ; long d
        MOV   C,bitvalue
        MOV   ?_function?BIT+1,C             ; bit  e
        LCALL _function
        MOV   intresult+0,R6                 ; store int
        MOV   intresult+1,R7                 ; retval
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Asssembly code for the example function:

NAME          MODULE                 ; Names of the program module
?PR?FUNCTION?MODULE  SEGMENT  CODE   ; Seg for prg code in 'function'
?DT?FUNCTION?MODULE  SEGMENT  DATA OVERLAYABLE
                                     ; Seg for local vars in 'function'
?BI?FUNCTION?MODULE  SEGMENT  BIT  OVERLAYABLE
                                     ; Seg for local bit vars in 'function'

PUBLIC        _function, ?_function?BYTE, ?_function?BIT
                                        ; Public symbols for 'C' function call

RSEG          ?PD?FUNCTION?MODULE    ; Segment for local variables
?_function?BYTE:                     ; Start of parameter passing segment
v_a:    DS   2                       ;   int variable: v_a
v_b:    DS   1                       ;   char variable: v_b
v_d:    DS   4                       ;   long variable: v_d
.
.                                    ; Additional local variables
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RSEG          ?BI?FUNCTION?MODULE    ; Segment for local bit variables
?_function?BIT:                      ; Start of parameter passing segment
v_c:    DBIT  1                      ;   bit variable: v_c
v_e:    DBIT  1                      ;   bit variable: v_e
.
.                                    ; Additional local bit variables
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RSEG          ?PR?FUNCTION?MODULE    ; Program segment
_function:    MOV  v_a,R6            ; A function prolog and epilog is
              MOV  v_a+1,R7          ; not necessary.  All variables can
              MOV  v_b,R5            ; immediately be accessed.
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              MOV  R6,#HIGH retval   ; Return value
              MOV  R7,#LOW  retval   ; int constant
              RET                    ; Return

Cx51 User's Guide

Small Model Example

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